The present invention relates generally to the field of time domain reflectometers, and more particularly to a line interface circuit that enables a time domain reflectometer to operate without a dead zone.
Time domain reflectometry is commonly used within the telephone and cable television industries. A Time Domain Reflectometer (xe2x80x9cTDRxe2x80x9d) sends a pulse down a transmission line and then monitors the transmission line for any reflections of that pulse. Reflections are caused by changes in impedance along the transmission line. A change in impedance may indicate the presence of a fault. As a pulse transmitted by a TDR reaches the impedance mismatch, a portion of the transmitted pulse is reflected back to the TDR. Because the transmitted and reflected pulses travels along the transmission line at a known speed of propagation, the exact location of the impedance mismatch may be determined by measuring the time at which the transmitted pulse is transmitted and the time at which the reflected pulse is received by the TDR.
The magnitude of the reflected pulse is proportional to the magnitude of the impedance mismatch. The sign or polarity of the reflected pulse is determined by the direction of the change in impedance. For example, if the transmitted pulse is positive and the impedance at the fault increases, then the reflected pulse will be positive. A break in the line will result in strong positive reflected pulse. If the transmitted pulse is positive and the impedance at the fault decreases, then the reflected pulse will be negative. For example, a short in the line will produce a negative reflected pulse. Thus, the nature of the fault may be determined or inferred from analysis of the reflected waveforms.
The energy of the transmitted pulse is dependent on the width of the pulse. The larger the pulse width, the more energy is transmitted and therefore the further the signal will travel down the line. Accordingly, many currently available TDRs have a limited number selectable pulse width settings. Each pulse setting produces pulses of substantially identical width. Thus, each pulse of a selected width has a substantially identical frequency spectrum, which can result in electromagnetic interference with digital services on the line.
Conventional TDR pulse generation technology uses low speed logic to generate pulses or analog RLC circuits to generate half-sine wave transmitted pulses. The rise time of conventionally generated pulses is relatively slow, thereby making it very difficult to interpret reflections from some types of faults such as water in the cable, bridge taps, untwisted cable, etc. The slow rise time problem is particularly acute when using long half-sine wave transmitted pulses.
Another problem with currently existing TDRs is they all have a xe2x80x9cdead zonexe2x80x9d in which reflected pulses arrive back at the TDR before the end of the transmitted pulse. In the dead zone, the reflected pulse is masked by the transmitted pulse. It is impossible to detect faults within the dead zone. The length of dead zone is determined by the width of the transmitted pulse. Thus, the wider the transmitted pulse, the longer the dead zone. Current TDR practice attempts to reduce the length of the dead zone by using short pulses or eliminate the dead zone by connecting the TDR to the system under test by test leads that are longer than the dead zone.
Briefly stated, the present invention provides a time domain reflectometer transmission line interface the has no dead zone. The line interface includes a pulse driver for generating a transmitted pulse signal. A coupling transformer coupled to said pulse driver couples the transmitted pulse signal to a transmission line and receives a reflected pulse signal from the transmission line. A differential amplifier is coupled to the pulse driver and to the coupling transformer through a network configured such that said transmitted pulse is balanced at the inverting and non-inverting inputs of the differential amplifier. Thus, the differential amplifier produces no output signal in response to the transmitted pulse signal and a time domain reflectometer including the interface of the present invention has no dead zone.